Power loading in MU-MIMO

ABSTRACT

Embodiments of a system and method for transmitting data from an access point in a multiple user multiple input multiple output (MU-MIMO) system are provided. A first indication of signal quality (ISQ) is received at the access point from a first station and a second ISQ is received from a second station. The access point sets a first power level and a first modulation and coding scheme (MCS) for transmission of a first aggregated media access control (MAC) protocol data unit (A-MPDU) to the first station as a function of the first ISQ and an amount of payload data corresponding to the first A-MPDU. The access point also sets a second power level and a second MCS for transmission of a second A-MPDU as a function of the second ISQ and an amount of payload data corresponding to the second A-MPDU.

BACKGROUND

Multiple-user multiple-input multiple-output (MU-MIMO) systems cantransmit and receive signals to/from multiple users at a single antennaarray at the same time. In a MU-MIMO system, multiple signals are sentin parallel and are kept separate from one another not by modulation orcoding techniques, but by transmitting (or receiving) each signal in adifferent (e.g., orthogonal) direction. The process of transmitting (orreceiving) a signal in a specific direction using an antenna array isknown as beamforming. By selecting a beam for each signal that willproduce limited interference with other parallel beams, multiple signalscan be transmitted or received at the same time. Beams are oftencalculated to be orthogonal to one another in order to minimize theinterference between the beams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a wireless communication system inaccordance with embodiments.

FIG. 2 illustrates an example of a mobile station for communicating inthe wireless communication system of FIG. 1 in accordance withembodiments.

FIG. 3 illustrates an example of an access point for communicating inthe wireless communication system of FIG. 1 in accordance withembodiments.

FIG. 4 illustrates an example frame for transmission by the access pointof FIG. 1 in accordance with embodiments.

FIG. 5 illustrates an example frame for transmission by the access pointof FIG. 1 using power loading in accordance with embodiments.

FIG. 6 illustrates an example block diagram for forming and transmittinga power loaded frame from the access point of FIG. 1 in accordance withembodiments.

FIG. 7 illustrates an example block diagram for forming and transmittinga long training signal from the access point of FIG. 1 using the powerloading applied in the frame of FIG. 6 in accordance with embodiments.

DETAILED DESCRIPTION

The following description and the drawings sufficiently illustratespecific embodiments to enable those skilled in the art to practicethem. Other embodiments may incorporate structural, logical, electrical,process, and other changes. Portions and features of some embodimentsmay be included in, or substituted for, those of other embodiments.Embodiments set forth in the claims encompass all available equivalentsof those claims.

In conventional systems, the lengths of a given mobile station's datastream can vary due to dynamic traffic conditions and the amount of datain the buffer. The transmission length of a given frame variesaccordingly and is determined by the longest data stream. To align theterminations of the different data streams, padding bits are added toshorter data streams until all data streams have the same length.Transmission of padding bits, however, wastes transmission power sincepower is used transmitting bits with no information, and also increasesthe packet error probability. Additionally, the transmission of extra(padding) bits can add to interference caused to other data streams andco-channel networks.

In accordance with embodiments, power loading of different data streamsmay be used to align terminations of the data streams with minimalpadding. For example, modulation and coding scheme (MCS) adjustments canbe made to some or all of the data streams to get the data streams tothe same length. Power loading can then be used to compensate for thechanges in MCS. In particular, for data streams that are shortened bymaking the MCS less robust, the transmission power for the data streamcan be increased to compensate for the less robust MCS. The overallpower can be maintained by reducing the power level and making the MCSmore robust for shorter data streams.

FIG. 1 illustrates an example of a wireless communication system 100.The wireless communication system 100 can include a plurality of mobilestations 102 in wireless communication with an access network 104. Theaccess network 104 forwards information between the mobile stations 102and another communications network 106. Communications network 106 caninclude the internet, a private intranet, or other network.

In an example, each mobile station 102 can include one or more antennas114 for transmitting and receiving wireless signals to/from an accesspoint 118 in the access network 104. The access point 118 can implementthe air interface to the mobile stations 102, and can transmit andreceive signals with an antenna array 116 coupled thereto. The accesspoint 118 can be communicatively coupled to the communications network106 for forwarding information to/from the mobile stations 106.

FIG. 2 illustrates an example of a mobile station 102. The mobilestation 102 can include a memory 202 for storage of instructions 204 forexecution on processing circuitry 206. The instructions 204 can comprisesoftware configured to cause the mobile station 102 to perform actionsfor wireless communication between the mobile station 102 and the accesspoint 118. The mobile station 102 can also include an RF transceiver 208for transmission and reception of signals with the antenna 114.

In some examples, the mobile station 102 can be a personal digitalassistant (PDA), a laptop or desktop computer with wirelesscommunication capability, a web tablet, a net-book, a wirelesstelephone, a wireless headset, a pager, an instant messaging device, adigital camera, an access point, a television, a medical device (e.g., aheart rate monitor, a blood pressure monitor, etc.), or other devicethat can receive and/or transmit information wirelessly.

FIG. 3 illustrates an example of an access point 118. The access point118 can include a memory 302 for storage of instructions 304 forexecution on processing circuitry 306. The instructions 304 can comprisesoftware configured to cause the access point 118 to perform actions forwireless communication between the mobile station 102 and the accesspoint 118. The access point 118 can also include an RF transceiver 308for transmission and reception of signals using the antenna array 116.The processing circuitry 306 can be configured to implement beamformingwith the antenna array 116. In an example, the processing circuitry 306can be configured to use the antenna array 116 to implement adaptivebeamforming in a MU-MIMO system. That is, multiple beams can beimplemented at the same time to different mobile stations 102. Moreover,the direction of each beam can change dynamically according to changesin the signal path to a given mobile station 102. The access point 118can include a network switch, router, or hub for sending and receivinginformation with the communications network 106.

In an example, the mobile station 102 and access point 118 can beconfigured to operate in accordance with one or more frequency bandsand/or standards profiles. For example, the mobile station 102 andaccess point 118 can be configured to communicate in accordance withspecific communication standards, such as the Institute of Electricaland Electronics Engineers (IEEE) standards. In particular, the mobilestation 102 can be configured to operate in accordance with one or moreversions of the IEEE 802.11ac communication standard for MU-MIMO Wi-Fi.

In some examples, RF transceiver 208 and RF transceiver 308 can beconfigured to transmit and receive orthogonal frequency divisionmultiplexed (OFDM) communication signals which comprise a plurality oforthogonal subcarriers. In broadband multicarrier examples, the mobilestation 102 and access point 118 can be configured to communicate inaccordance with an orthogonal frequency division multiple access (OFDMA)technique.

In other examples, the mobile station 102 and access point 118 can beconfigured to communicate using one or more other modulation techniquessuch as spread spectrum modulation (e.g., direct sequence code divisionmultiple access (DS-CDMA) and/or frequency hopping code divisionmultiple access (FH-CDMA)), time-division multiplexing (TDM) modulation,and/or frequency-division multiplexing (FDM) modulation.

FIG. 4 illustrates an example physical layer (PHY) frame 400 fortransmission by the access point 118 to multiple mobile stations 102 ina MU-MIMO implementation. In an example, the frame 400 is structured inaccordance with the IEEE 802.11ac standard. The frame 400 includes oneor more legacy training fields 406, legacy signaling fields 408, as wellas one or more very high throughput (VHT) training fields 410, and oneor more VHT signaling fields 412. The frame 400 also includes one ormore frame headers 414 a-c followed by one or more aggregated mediaaccess control (MAC) protocol data units (A-MPDU) 416 a-c, and one ormore tails 418 a-c. The transmission of frame 400 can be, for example,over a single 20 Mhz primary channel and up to seven 20 Mhz channelsaccording to the IEEE 802.11ac standard.

The training fields 406 and 410 can be used to transmit training signalsto the mobile stations 102 for use by the mobile stations 102 to setparameters in their receivers and decoders. In some examples, thetraining fields 406, 410 can include long and/or short training fields.The signaling fields 408 and 412 can be used to transmit informationregarding the decoding of the A-MPDUs 416 a-c. For example, thesignaling fields 408, 412 can contain information regarding themodulation and coding schemes (MCS) used to transmit the A-MPDUs 416a-c. The mobile stations 102 can then use the MCS information toproperly receive and decode the A-MPDU 416 a-c intended for them.

The A-MPDUs 416 a-c comprise the encoded payload data for the frame 400.Here, the payload data for a given A-MPDU 416 a-c comprises a pluralityof MPDUs generated by the MAC layer. As shown, the frame 400 includesthree A-MPDUs 416 a-c, each A-MPDU 416 a-c to be transmitted to adifferent mobile station 102. Accordingly, each A-MPDU 416 a-c cancomprise a plurality of MPDUs 416 a-c generated from a data streamintended for a single mobile station 102.

In an example, the A-MPDUs 416 a-c (as well as training fields 406, 410,frame headers 414 a-c, tails 418 a-c, signaling fields 408, 412),however, can be transmitted using beamforming to direct each A-MPDU 416a-c towards its intended recipient mobile station 102. Moreover, eachA-MPDU 416 a-c can be transmitted with a distinct beam such thatmultiple A-MPDUs 416 a-c can be transmitted in parallel on the same setof frequency subcarriers. Furthermore, since each A-MPDU 416 a-c can beindependently transmitted in a distinct beam, each A-MPDU 416 a-c canuse a different MCS. Accordingly, the frame 400 of FIG. 4 illustratestime in the horizontal access and precoding in the vertical access suchthat different A-MPDUs 416 a-c are transmitted on the same set offrequency subcarriers, but use different precoding to be transmitted ondifferent beams to different mobile stations.

As shown, the frame 400 comprises a conventional frame transmittedwithout power loading. Accordingly, MAC layer padding bits 420 are usedto align the terminations of each A-MPDU 416 a-c. In an example, the MAClayer padding bits 420 can align the terminations to within one byte andphysical layer padding bits 422 can be used to get exact bit alignmentbetween the A-MPDUs 416 a-c. As can be seen, the full transmission powerof the access point 118 is applied on useful data for a portion of theframe 400, but while the MAC layer padding bits 420 are beingtransmitted, some of the transmission power is used to transmit thepadding bits. Accordingly, only part of the transmission power is usedon the useful data.

FIG. 5 illustrates another example frame 500 where power loading is usedto align the terminations of the A-MPDUs 516 a-c. Frame 500 includes thesame fields as frame 400; legacy signaling fields 408, as well as one ormore very high throughput (VHT) training fields 410, and one or more VHTsignaling fields 412. Also the same as frame 400, frame 500 includes oneor more frame headers 514 a-c followed by one or more aggregated mediaaccess control (MAC) protocol data units (A-MPDU) 516 a-c, and one ormore tails 518 a-c. Similar to frame 400, the transmission of frame 500can be, for example, over a single 20 Mhz primary channel and up toseven 20 Mhz channels according to the IEEE 802.11ac standard.Additionally, the frame 500 of FIG. 5 illustrates time in the horizontalaccess and precoding in the vertical access such that different A-MPDUs516 a-c are transmitted on the same set of frequency subcarriers, butuse different precoding to be transmitted on different beams todifferent mobile stations.

For comparison purposes, in the example frame 500, the A-MPDUs 516 a-care formed from the same payload data as the A-MPDUs 416 a-c in frame400. The A-MPDUs 516 a-c, however, have terminations that are alignedwith much less MAC layer padding bits 520 than the MAC layer paddingbits 420 in frame 400. This is because the A-MPDUs 516 a-c are powerloaded and have their MCSs adjusted in order to align the terminations.Additionally, the overall length of frame 500 is less than frame 400 dueto the power loading and MCS adjustment used. In an example, physicallayer padding bits 522 can be used in frame 500 for padding less thanone byte in length.

For comparison purposes, the A-MPDU 516 a is formed from the samepayload data as used to form A-MPDU 416 a. Likewise, the A-MPDU 516 b isformed from the same payload data as used to form A-MPDU 516 b, and theA-MPDU 516 c is formed from the same payload data as used to form A-MPDU516 c. As shown the A-MPDU 416 a is the longest of the A-MPDUs 416 a-cof frame 400. Thus, the length of the frame 400 is determined in part bythe length of A-MPDU 416 a. The A-MPDU 416 b is in the middle for lengthand the A-MPDU 416 c is the shortest of the three A-MPDUs 416 a-c.

The frame 400 can be treated as a hypothetical frame generated by theaccess point 116. The hypothetical frame 400 can be used to determinethe power loading and MCS to apply to the A-MPDUs in order to align theterminations of the A-MPDUs. In this hypothetical frame 400, each A-MPDU416 a-c is generated to be transmitted at the same power level. Notably,however, the MCS of each A-MPDU 416 a-c is determined based on a signalquality between the access point 118 and the mobile station 102 to whichthe respective A-MPDU416 a-c would be transmitted. In this example, theA-MPDU 416 a is intended for a first mobile station 102 and the signalquality between the access point 118 and the first mobile station 102 ispoor, a more robust MCS would be used for the A-MPDU 416 a. The A-MPDU416 c is intended for a third mobile station 102 having a good signalquality and the A-MPDU 416 c has a correspondingly less robust MCS. TheA-MPDU 416 b is intended for a second mobile station 102 having a signalquality in-between the first and third mobile stations 102, and the MCSfor the A-MPDU 416 b is correspondingly in-between robustness of theA-MPDU 416 a and 416 c.

The signal quality between the access point 118 and a mobile station 102can correspond to the downlink (e.g., from the access point 118 to themobile station 102), uplink (e.g., from the mobile station 102 to theaccess point 118) or a combination (e.g., an average) of the downlinkand uplink signal qualities. In an example, the signal quality is basedon an indication received from a mobile station 102 regarding the signalquality of a downlink signal sent from the access point 118. Forexample, the access point 118 can broadcast a sounding signal that canbe received by one or more mobile stations 102. A mobile station 102receiving the sounding signal can send a return signal having anindication of the signal quality of the sounding signal as received bythe mobile station 102. In an example, the return signal from the mobilestation 102 can provide a signal-to-noise ratio for the sounding signal.In another example, the return signal can provide a bit-error rate forthe sounding signal. In still other examples, the mobile station 102 canprocess the sounding signal and determine a preferred MCS to be used bythe access point 118. The mobile station 102 can then send theappropriate MCS to the access point 118 as a preferred MCS for theaccess point 118 to use for A-MPDU transmission to the mobile station102.

In any case, in frame 400, when the A-MPDUs 416 a-c are assigned to betransmitted with equal power, but the signal quality between the accesspoint 118 and respective mobile stations 102 varies, the length of theA-MPDUs 416 a-c will also vary. Accordingly, the A-MPDU 416 a is thelongest A-MPDU 416 a-c since the A-MPDU corresponds to the mobilestation 102 having the poorest signal quality and the access point 118,therefore, applies the most robust MCS to A-MPDU 416 a as compared tothe A-MPDUs 416 b and 416 c.

In order to align the terminations of the A-MPDUs 516 a-c, the power isincreased above the power level used for the A-MPDUs 416 a-c in frame400 for long A-MPDUs and the power level is decreased below the powerlevel used in frame 400 for short A-MPDUs. Accordingly, the A-MPDU 516 ahas a higher power level than the A-MPDU 416 a, the A-MPDU 516 c has alower power level than the A-MPDU 416 c, and the A-MPDU 516 b has thesame power level as the A-MPDU 416 b to maintain the same length.

Changing the power level enables the MCS to be changed which changes thelength of an A-MPDU. Accordingly, in order to decrease the length of theA-MPDU 416 a, the power level is increased which compensates for adecrease in the robustness of the MCS. With the higher power, the MCScan be made less robust which shortens the length of the A-MPDU 416 asuch that A-MPDU 516 a is generated. Since the power level for theA-MPDU 516 a was increased above the power level assigned to the A-MPDUs416 a-c, the total power (without any other changes) transmitted by theaccess point 118 would increase correspondingly. In order to maintainthe same total power as frame 400, the power level for A-MPDU 416 c canbe decreased since the A-MPDU 416 c has a short length. In order tocompensate for the decreased power, the MCS for the A-MPDU 416 c is mademore robust and the length increases such that the A-MPDU 516 c isgenerated. In this example, the A-MPDU 416 b does not have its lengthchanged, and the power level and MCS for 516 b are the same as that forA-MPDU 416 b.

In an example, the amount of increase in power level for the A-MPDU 516a (as compared to the A-MPDU 416 a) can correspond to the amount ofincrease in robustness required for the MCS of the A-MPDU 516 a in orderto align the termination of the A-MPDU 516 a with the terminations ofthe A-MPDU 516 b and 516 c. Thus, if a large increase in the MCS isneeded a large power increase can also be applied. In an example, therelationship between the amount of MCS change per power level change isbased on the structure of the receiver in the mobile station 102 (e.g.,based on amount of power received). In an example, a 3 dB in the powerlevel increase corresponds to 1-2 level decrease in MCS, where there area total of 8 MCS levels (2-3 dB per MCS level).

In an example, the access point 118 determines the power levels and MCSsto be applied to each of the A-MPDUs 516 a-c together. Accordingly, theaccess point 118 can manage the total power limiting the total increasein power on A-MPDUs 516 a-c to equal the total decrease in power onother A-MPDUs 516 a-c transmitted in the same frame. Moreover, theaccess point 118 can manage the MCSs for each A-MPDU 516 a-c such thatthe length of each A-MPDU 516 a-c is substantially the same. In anexample, each A-MPDU 516 a-c is the same length to within one byte.Notably, the length of each A-MPDU 516 a-c is dependent upon not onlythe MCS, but also the amount of payload data provided by the MAC layerfor the A-MPDU 516 a-c. Moreover, since transmission power is not wastedon (as many) MAC layer padding bits 420, the overall length of the frame500 can be reduced as compared to the frame 400.

In some examples, the access point 118 can send a notification of thelength of and the MCS used for a given A-MPDU 516 a-c in one or more ofthe VHT-signaling fields 412 corresponding to the given A-MPDU 516 a-c.Accordingly, in some examples, the power loading and MCS adjustment canbe transparent to the mobile station 102. The mobile station 102 merelyreceives the MCS used for the A-MPDU 516 a-c to be received, and decodesthe A-MPDU 516 a-c using the MCS provided. Whether the MCS was increasedor decreased is unknown and not needed by the mobile station 102.Additionally, the mobile station 102 does not need to know the powerlevel for the A-MPDU 516 a-c.

In an example, the power level and MCS for each A-MPDU in a frame areset on a frame-by-frame basis. Accordingly, a hypothetical length can bedetermined for all the A-MPDUs in a first frame. The hypothetical lengthcan be determined using the same power level for each A-MPDU and thepreferred MCSs received from the mobile stations 102 corresponding tothe A-MPDUs. Based on the amount of data for each A-MPDU, thehypothetical length can be determined. These lengths can then beadjusted as described above by changes in power level and MCS one ormore of the A-MPDUs. The same process can be repeated for the next framebased on the new payload data for the new A-MPDUs and the mobilestations 102 corresponding to these new A-MPDUs.

FIG. 6 illustrates an example block diagram 600 for forming andtransmitting the A-MPDUs 516 a-c with power loading to align theterminations of the A-MPDUs 516 a-c. Block diagram 600 illustratesforming A-MPDUs that correspond to the IEEE 802.11ac standard. At blocks602 a-n data streams (e.g., payload data from the MAC layer) forrespective mobile stations a-n have respective MCSs applied in order toalign terminations as describe above with respect to frame 500. Next,the data streams have a cyclic prefix added at block 604 in accordancewith an orthogonal frequency multiplexing (OFDM) modulation scheme. Atblock 606, the K diagonal matrix is applied that corresponds to thepower loading. The power level applied each data stream corresponds tothe MCSs that was applied at block 602 a-n for the respective datastream. At block 608, independent beams are formed for each data streamwith a Q matrix that applies pre-coding to the data streams. The Qmatrix comprises a plurality of Q vectors that are orthogonal to eachother; one Q vector for each beam and each mobile station 102. At blocks610 a-n, each data stream processed by an inverse Fourier transform.Each data stream is then transmitted from the antenna array 116 with itsbeam and power level applied at blocks 606 and 608. Accordingly, thepower level is set for each beam.

In an example, the power level within a data stream transmitted to agiven mobile station a-n remains the same throughout the frame 500. Forexample, the VHT long training field 510 and/or other fields withinframe 500 can be transmitted using a power level that matches thecorresponding A-MPDU 516 a-c. According, a first VHT long training field510 intended for a first mobile station 102 can be transmitted at thepower level set for the A-MPDU 516 a which is also intended for thefirst mobile station 102. FIG. 7 illustrates an example block diagram700 for forming and transmitting the VHT long training field 510 at thesame power level as associated A-MPDUs 516 a-c.

At block 702, a long training signal is input for the long trainingfield 510. At blocks 704 a-n, the long training signal is mapped intomultiple data streams each having the long training signal for a givenmobile station a-n. The mobile stations a-n with respect to the longtraining signal in block diagram 700 are the same mobile stations a-nfor the A-MPDUs generated with block diagram 600. Since a single longtraining signal is mapped to all the mobile stations a-n, each longtraining signal has the same MCS applied thereto. In an example, eachlong training signal has the most robust MCS applied thereto in order toensure accurate reception at the mobile stations a-n. At block 706, acyclic prefix is added to the long training signal in accordance with anOFDM modulation scheme. At block 708, the respective power levels foreach data stream are applied. The power levels applied at block 708 arethe same as the power levels applied at block 606 of block diagram 600.At block 710, beams are formed for the data streams and, at block 712,inverse Fourier transforms are applied before transmitting the longtraining signals with the antenna array 116.

Embodiments may be implemented in one or a combination of hardware,firmware and software. Embodiments may also be implemented asinstructions stored on a computer-readable medium, which may be read andexecuted by at least one processing circuitry to perform the operationsdescribed herein. A computer-readable medium may include any mechanismfor storing in a form readable by a machine (e.g., a computer). Forexample, a computer-readable medium may include read-only memory (ROM),random-access memory (RAM), magnetic disk storage media, optical storagemedia, flash-memory devices, and other storage devices and media.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b)requiring an abstract that will allow the reader to ascertain the natureand gist of the technical disclosure. It is submitted with theunderstanding that it will not be used to limit or interpret the scopeor meaning of the claims. The following claims are hereby incorporatedinto the detailed description, with each claim standing on its own as aseparate embodiment.

What is claimed is:
 1. A method for transmitting data from an accesspoint in a multiple user multiple input multiple output (MU-MIMO)system, the method comprising: receiving a first indication of signalquality (ISQ) from a first station; receiving a second ISQ from a secondstation; setting a first power level and a first modulation and codingscheme (MCS) for transmission of a first aggregated media access control(MAC) protocol data unit (A-MPDU) to the first station as a function ofthe first ISQ and an amount of payload data corresponding to the firstA-MPDU; setting a second power level and a second MCS for transmissionof a second A-MPDU as a function of the second ISQ and an amount ofpayload data corresponding to the second A-MPDU, the amount of payloaddata corresponding to the first A-MPDU and the amount of a payload datacorresponding to the second A-MPDU are different, and the first andsecond power levels and the first and second MCSs are set to produce asimilar length for the first A-MPDU and the second A-MPDU; transmittingthe first A-MPDU at the first power level and within a first frame tothe first station; and transmitting the second A-MPDU at the secondpower level and within the first frame to the second station.
 2. Themethod of claim 1, wherein the first and second power levels and thefirst and second MCSs are selected such that the length of the firstA-MPDU is within one byte of the length of the second A-MPDU.
 3. Themethod of claim 1, comprising: calculating a first hypothetical lengthcorresponding to the first A-MPDU based on the payload datacorresponding to the first A-MPDU encoded using a first hypotheticalMCS, the first hypothetical MCS corresponding to the first ISQ;calculating a second hypothetical length corresponding to the secondA-MPDU based on the payload data corresponding to the second A-MPDUencoded using a second hypothetical MCS, the second hypothetical MCScorresponding to the second ISQ; and responsive to determining that thefirst hypothetical length is longer than the second hypothetical length,setting at least one of: the first MCS at a less robust rate than thefirst hypothetical MCS, and the second MCS at a more robust rate thanthe second hypothetical MCS.
 4. The method of claim 3, wherein the firstand second hypothetical MCSs correspond to respective signalstransmitted to the first and second stations at a third power level. 5.The method of claim 4, comprising: responsive to determining that thefirst hypothetical length is longer than the second hypothetical length,setting at least one of: the first power level higher than the thirdpower level, and the second power level lower than the third powerlevel, when the first hypothetical length is longer than the secondhypothetical length.
 6. The method of claim 4, wherein the first ISQcorresponds to a received signal quality at the first station of asignal transmitted by the access point at the third power level; andwherein the second ISQ corresponds to a received signal quality at thesecond station of the signal transmitted by the access point at thethird power level.
 7. The method of claim 1, wherein the first ISQcorresponds to a preferred MCS received from the first station; andwherein the second ISQ corresponds to a preferred MCS received from thesecond station.
 8. The method of claim 1, comprising: adjusting on aframe-by-frame basis the power level and MCS for A-MPDUs transmitted tothe first station as a function of the first ISQ and the amount ofpayload data corresponding to the A-MPDUs to be transmitted to the firststation; adjusting on a frame-by-frame basis the power level and MCS forA-MPDUs transmitted to the second station as a function of the secondISQ and the amount of payload data corresponding to the A-MPDUs to betransmitted to the second station, the power level and MCSs for A-MPDUstransmitted to the first station and the second station being set toproduce similar length A-MPDUs transmitted to the first and secondstations in each frame.
 9. The method of claim 8, wherein A-MPDUs havinga shorter calculated hypothetical length as compared to other A-MPDUs inthe same frame, are transmitted at a lower power level and a more robustMCS, and wherein A-MPDUs having a longer calculated hypothetical lengthas compared to other A-MPDUs in the same frame, are transmitted at ahigher power level and a less robust MCS.
 10. The method of claim 8,wherein the total power from frame-to-frame remains substantiallyconstant.
 11. An apparatus for transmitting signals in a multiple usermultiple input multiple output (MU-MIMO) system, the apparatuscomprising: a processor configured to: receive a first indication ofsignal quality (ISQ) from a first station; receive a second ISQ from asecond station; set a first power level and a first MCS for transmissionof a first aggregated media access control (MAC) protocol data unit(A-MPDU) to the first station as a function of the first ISQ and anamount of payload data corresponding to the first A-MPDU; and set asecond power level and a second MCS for transmission of a second A-MPDUas a function of the second ISQ and an amount of payload datacorresponding to the second A-MPDU, the amount of payload datacorresponding to the first A-MPDU and the amount of payload datacorresponding to the second A-MPDU are different and the first andsecond power levels and the first and second MCSs are set to produce asimilar length for the first A-MPDU and the second A-MPDU.
 12. Theapparatus of claim 11, where the processor is configured to: calculate afirst hypothetical length corresponding to the first A-MPDU based on thepayload data corresponding to the first A-MPDU encoded using a firsthypothetical MCS, the first hypothetical MCS corresponding to the firstISQ; calculate a second hypothetical length corresponding to the secondA-MPDU based on the payload data corresponding to the second A-MPDUencoded using a second hypothetical MCS, the second hypothetical MCScorresponding to the second ISQ; and responsive to a determination thatthe first hypothetical length is longer than the second hypotheticallength, set at least one of: the first MCS at a less robust rate thanthe first hypothetical MCS, and the second MCS at a more robust ratethan the second hypothetical MCS.
 13. The apparatus of claim 12, whereinthe processor is configured to: responsive to the determination that thefirst hypothetical length is longer than the second hypothetical length,set at least one of: the first power level higher than the third powerlevel, and the second power level lower than the third power level. 14.The apparatus of claim 11, wherein the processor is configured to:adjust on a frame-by-frame basis the power level and MCS for A-MPDUstransmitted to the first station as a function of the first ISQ and theamount of payload data corresponding to the A-MPDUs to be transmitted tothe first station; adjust on a frame-by-frame basis the power level andMCS for A-MPDUs transmitted to the second station as a function of thesecond ISQ and the amount of payload data corresponding to the A-MPDUsto be transmitted to the second station, the power level and MCSs forA-MPDUs transmitted to the first station and the second station beingset to produce similar length A-MPDUs transmitted to the first andsecond stations in each frame.
 15. A method for transmitting data froman access point in a multiple user multiple input multiple output(MU-MIMO) system, the method comprising: determining a first power leveland a first MCS for transmission of a first aggregated media accesscontrol (MAC) protocol data unit (A-MPDU) as a function of an amount ofpayload data for the first A-MPDU; determining a second power level anda second MCS for transmission of a second A-MPDU as a function of anamount of payload data for the first A-MPDU, wherein the first andsecond power levels and the first and second MCSs are set to produce asimilar length for the first A-MPDU and the second A-MPDU; generating afirst A-MPDU using a first MCS; generating a second A-MPDU using asecond MCS; transmitting the first A-MPDU at the first power level; andtransmitting the second A-MPDU at the second power level.
 16. The methodof claim 15, wherein the first A-MPDU is transmitted to a first stationusing a first beam direction on an antenna array and the second A-MPDUis transmitted to a second station using a second beam direction on theantenna array.
 17. The method of claim 16, comprising: receiving a firstindication of signal quality (ISQ) from a first station, wherein thefirst power level and the first MCS are determined as a function of acombination of the first ISQ and the amount of payload data for thefirst A-MPDU; and receiving a second ISQ from a second station, whereinthe second power level and the second MCS are determined as a functionof a combination of the second ISQ and the amount of data for the secondA-MPDU.
 18. The method of claim 15, comprising: adjusting on aframe-by-frame basis the power level and MCS for A-MPDUs as a functionof the ISQ and the amount of payload data corresponding to the A-MPDUs.